This invention relates to a material which exhibits excellent blood compatibility. More particularly, it relates to a blood-compatible material suitable for small-diameter vascular prostheses.
Vascular prostheses are examples of materials which must be compatible with blood. However, vascular prostheses currently available are prepared from polyester knitted web or porous polytetrafluoroethylene and exhibit insufficient blood compatibility, so that they are usable only for arteries of a large diameter wherein thrombotic occlusion rarely occurs. When these vascular prostheses are employed for arteries of a small diameter or for veins, thrombosis and resultant occlusion often would occur within a short period of time.
Various materials having improved blood compatibility have been proposed as will be described hereinafter. However, each of them is somewhwat unsuitable for use in vascular prostheses of small diameter. In Japanese Patent Publication No. 42759/1971 and Japanese Patent Laid-Open Nos. 150793/1975 and 24651/1976, blood-compatible materials which are prepared from hydrogel obtained by crosslinking hydrophilic polymers such as poly(2-hydroxyethyl methacrylate) and polyvinyl alcohol are disclosed. However, these materials cannot be applied to vascular prostheses because of their insufficient blood compatibility and low strength. Japanese Patent Laid-Open Nos. 125493/1974 and 125978/1976 disclose processes for improving blood compatibility by graft polymerizing the above mentioned hydrophilic polymer(s) onto the surface of a base material. The blood-compatible material which is prepared according to the foregoing processes with the use of an appropriate base material would exhibit satisfactory tensile strength. However, the blood compatibility thereof is similar to those of the hydrophilic polymers. Therefore thrombosis and resulting occlusion would occur within a short period of time if this material were used for vascular prostheses of small diameter.
In addition, Japanese Patent Laid-Open Nos. 66187/1973 and 106778/1978 disclose processes for imparting blood compatibility by fixing anticoagulants such as heparin or urokinase onto the surface of certain materials. The blood compatibility of a material thus obtained is high at the beginning but gradually decreases so that it is impossible to obtain a stable, satisfactory blood compatibility level. This instability may be caused by a decrease in the anticoagulant effect resulting from gradual denaturation of the anticoagulants fixed on the surface of the material. These processes have another disadvantage in the high cost of the anticoagulants themselves as well as the expense required for the fixing treatment.
Thus, despite the numerous proposals as described above, no material which exhibits excellent blood compatibility for a long period of time is available for vascular prostheses of small diameter.